Performance metrics collection in Selenium Python - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Extracting page load time using Selenium and Performance API

You want to measure the page load time of a website using Selenium in Python. Which code snippet correctly extracts the loadEventEnd and navigationStart timing values and calculates the load time in milliseconds?

Selenium Python

from selenium import webdriver

driver = webdriver.Chrome()
driver.get('https://example.com')

# Your code here to get load time

load_time = driver.execute_script('return window.performance.timing.loadEventEnd - window.performance.timing.navigationStart;')
print(load_time)
driver.quit()

A load_time = driver.execute_script('return window.performance.timing.domComplete - window.performance.timing.domLoading;')

B load_time = driver.execute_script('return window.performance.timing.loadEventEnd - window.performance.timing.navigationStart;')

C load_time = driver.execute_script('return window.performance.timing.responseEnd - window.performance.timing.requestStart;')

D load_time = driver.execute_script('return window.performance.timing.fetchStart - window.performance.timing.navigationStart;')

💡 Hint

Look for the difference between the event when the page finishes loading and when navigation started.

The loadEventEnd marks when the load event finished, and navigationStart marks when navigation started. Their difference gives the total page load time.

🧠 Conceptual

intermediate

Understanding Navigation Timing API properties

Which of the following window.performance.timing properties represents the moment when the browser starts fetching the document?

A fetchStart

B navigationStart

C domLoading

D responseStart

💡 Hint

Think about when the browser actually begins to request the document from the server.

fetchStart marks the time when the browser starts fetching the document, including any redirects. navigationStart is when navigation begins but may be before fetching.

📝 Syntax

advanced

Correctly retrieving First Contentful Paint (FCP) metric

You want to get the First Contentful Paint (FCP) metric using Selenium's execute_script method. Which code snippet correctly retrieves FCP from the Performance API?

Selenium Python

fcp = driver.execute_script('''
  const perfEntries = performance.getEntriesByType('paint');
  for (const entry of perfEntries) {
    if (entry.name === 'first-contentful-paint') {
      return entry.startTime;
    }
  }
  return -1;
''')

A fcp = driver.execute_script('return performance.getEntriesByType("paint")[0].startTime;')

B fcp = driver.execute_script('return performance.getEntriesByName("first-contentful-paint")[0].startTime;')

C fcp = driver.execute_script('return performance.timing.firstContentfulPaint;')

D fcp = driver.execute_script('return performance.getEntriesByType("paint").find(e => e.name === "first-contentful-paint").startTime;')

💡 Hint

Use getEntriesByType('paint') and find the entry with name first-contentful-paint.

The Performance API's paint entries include 'first-contentful-paint'. Using find on getEntriesByType('paint') is the correct way to get its startTime. Option D fails because getEntriesByName is not guaranteed to work for paint entries. Option D is invalid because performance.timing does not have firstContentfulPaint. Option D returns the first paint entry which may be 'first-paint', not 'first-contentful-paint'.

🔧 Debug

advanced

Diagnosing error when accessing Performance API in Selenium

You run this code to get navigation timing but get an error: JavascriptException: window.performance is undefined. What is the most likely cause?

Selenium Python

timing = driver.execute_script('return window.performance.timing')

A The page has not fully loaded before the script runs, so <code>window.performance</code> is not available yet.

B The browser driver does not support the Performance API.

C The URL loaded is a blank page or about:blank, which has no performance data.

D You need to enable browser performance logging in Selenium options before accessing <code>window.performance</code>.

💡 Hint

Check what page is loaded before accessing performance data.

If the page is about:blank or empty, window.performance may be undefined or have no timing data. The Performance API is standard and available in modern browsers, so B is unlikely. A is incorrect because window.performance exists early. C is the most likely cause when the page is blank.

❓ optimization

expert

Efficiently collecting multiple performance metrics in one script

You want to collect loadEventEnd, domContentLoadedEventEnd, and first-contentful-paint metrics in a single Selenium execute_script call. Which code snippet correctly returns all three as a dictionary?

A

return {
  loadEventEnd: performance.timing.loadEventEnd - performance.timing.navigationStart,
  domContentLoadedEventEnd: performance.timing.domContentLoadedEventEnd - performance.timing.navigationStart,
  fcp: performance.getEntriesByType('paint').filter(e => e.name === 'first-contentful-paint')[0].startTime
}

B

return {
  loadEventEnd: window.performance.timing.loadEventEnd,
  domContentLoadedEventEnd: window.performance.timing.domContentLoadedEventEnd,
  fcp: window.performance.getEntriesByName('first-contentful-paint')[0].startTime
}

C

return {
  loadEventEnd: performance.timing.loadEventEnd,
  domContentLoadedEventEnd: performance.timing.domContentLoadedEventEnd,
  fcp: performance.getEntriesByType('paint').find(e => e.name === 'first-contentful-paint').startTime
}

D

return {
  loadEventEnd: performance.timing.loadEventEnd,
  domContentLoadedEventEnd: performance.timing.domContentLoadedEventEnd,
  fcp: performance.getEntriesByType('paint')[0].startTime
}

💡 Hint

Calculate relative times from navigationStart and correctly find the FCP entry.

Option A correctly returns relative times for loadEventEnd and domContentLoadedEventEnd by subtracting navigationStart, which is common practice for meaningful metrics. It also uses filter to find the 'first-contentful-paint' entry safely. Option A returns absolute timestamps which are less useful. Option A uses getEntriesByName which may not work for paint entries. Option A returns the first paint entry which may not be FCP.